Anomalous behaviour in the layer halides and oxyhalides of titanium and vanadium: a study of materials close to delocalisation

Crystal structure evolution in the van der Waals vanadium trihalides

Most transition-metal trihalides are dimorphic. The representative chromium-based triad, CrCl₃, CrBr₃, CrI₃, is characterized by the low-temperature (LT) phase adopting the trigonal BiI₃-type while the structure of the high-temperature (HT) phase is monoclinic of AlCl₃type (C2/m). The structural transition between the two crystallographic phases is of the first-order type with large thermal hysteresis in CrCl₃and CrI₃. We studied crystal structures and structural phase transitions of vanadium-based counterparts VCl₃, VBr₃, and VI₃by measuring specific heat, magnetization, and x-ray diffraction as functions of temperature and observed an inverse situation. In these cases, the HT phase has a higher symmetry while the LT structure reveals a lower symmetry. The structural phase transition between them shows no measurable hysteresis in contrast to CrX₃. Possible relations of the evolution of the ratioc/aof the unit cell parameters, types of crystal structures, and nature of the structural transitions in V and Cr trihalides are discussed.

Synthesis and Crystal Structure of the Layered Lanthanide Oxychlorides Ba3Ln2O5Cl2

Single crystals of a new family of layered lanthanide oxychlorides, Ba₃Ln₂O₅Cl₂ (Ln = Gd-Lu), have been synthesized from a molten barium flux. This family crystallizes in the space group I4/mmm (No. 139; Z = 2) with lattice parameters a = 4.3384(1)-4.4541(1) Å and c = 24.5108(7)-24.8448(9) Å. Ba₃Ln₂O₅Cl₂ phases are built up of two different blocks: a perovskite double layer of stoichiometry Ba₂Ln₂O₅ formed by corner-connected LnO₅ tetragonal bipyramids and a puckered rock-salt-like interlayer of composition BaCl₂. A complete structural study along with bond-valence-sum calculations shows that, for lanthanides larger than gadolinium, the structure becomes unstable. Density functional theory calculations show that the valence-band edge is dominated by oxygen orbitals, whereas the conduction band forms from Ba 5d orbitals. The synthesis of this family suggests a route to other potential multianion phases.